This invention relates to rotary positioning devices of the type wherein a shaft can be commanded to rotate to any one of a predetermined number of fixed angular positions. In the preferred application, the invention relates to electromagnetic wheel indicators of the type having symbols marked upon the drum of a rotor that can turn to bring any of the symbols to a display station.
In typical electromagnetic wheel indicators, the angular orientation of a permanent magnet rotor is governed by an electromagnetic stator which is capable of establishing, upon receiving appropriate command signals, any one of a plurality of differently oriented magnetic fields. As the stator field is changed to a new orientation, the rotor appropriately realigns itself therewith and thus the rotor is capable of assuming any one of a plurality of discrete angular positions. Typically, attached to the rotor is a symbol-bearing wheel and at each particular rotor angle position a particular symbol registers in a display station. Such devices are well known and understood in the art. See for instance U.S. Pat. Nos. 3,411,155, 3,636,557, and 3,636,550.
A common problem with such devices is that of performance variation with temperature changes. More specifically, such devices frequently provide sluggish or overdamped response at cold temperatures while providing underdamped response at room or higher temperatures. Previous attempts to solve such problem have included adjustments in lubricant viscosity and/or adjustments and magnetic strength.